(1) Field of the Invention
The present invention relates to a technique for automatically restoring an optical output that has been interrupted or the like by an occurrence of a line fault in an optical communication. In particular, it relates to an automatic power restoring method for when an optical transmission path includes an optical amplification medium, and an optical communication system to which the method is applied.
(2) Related Art
For example, international standards such as those of the IEC (International Electrotechnical Commission) require the optical power of a high power communication laser light to be reduced to a safelight level (class 1: average power+10 dBm or less) within a prescribed time after a line fault occurs, and these standards are also applied to various optical communication systems such as existing unrepeatered transmission systems and the like.
The abovementioned unrepeatered transmission system is a system for transmitting a signal light sent out from a transmitting section to an optical transmission path, to a receiving section without using a repeater apparatus. Furthermore, unrepeatered transmission systems in which a long distance transmission is achieved by applying Raman amplification or optical amplification of remote pumping system have been practically used. A large number of optical transmission techniques applied with Raman amplification and the like have been proposed both with and without repeater apparatuses. As specific examples, there are well known a technique relating to a power control of pumping light and amplification light (refer to Japanese Unexamined Patent Publication No. 2001-251006 and Japanese Unexamined Patent Publication No. 2002-57624), a technique relating to a wavelength dependence control of gain by Raman amplification (refer to Japanese Unexamined Patent Publication No. 2001-223646 and Japanese Unexamined Patent Publication No. 2001-249369) and the like.
In a conventional unrepeatered transmission system as described above, in the case where a line fault occurs due to disconnection or the like of an optical cable used for an optical transmission path, an automatic power shutdown (APSD) method for automatically detecting the occurrence of the fault and stopping operations of the transmitting section and receiving section (in the case where the receiving section includes a pumping light source for performing Raman amplification or optical amplification of remote pumping system) is applied to prevent a high level laser light from being emitted to outside, to thereby ensure the safety. In such a system to which APSD is applied, for example, a supervisory control signal light (Optical Supervisory Channel: OSC) having a transmission rate of about 1 Mbps, which is set to the safelight level of class 1 transmitted from the transmitting section, is utilized to perform processing such as, an automatic line fault detection, an automatic continuity detection, and stopping/restarting of the transmitting section and receiving section.
However, in the conventional unrepeatered transmission system as described above in which Raman amplification and optical amplification of remote pumping system are applied, since the transmission distance is long, considerable losses occur on the optical transmission path. Therefore, when a line fault occurs, it becomes difficult to receive a supervisory control signal light (optical power at a safelight level with a transmission rate of approximately 1 Mbps) sent out from the transmitting section as described above. Consequently, there is a problem in that in order to enable the reliable detection of continuity by the dissolution of line fault, a burden on the circuit design of the receiving section becomes significantly larger.
As a specific example, the case of the unrepeatered transmission system applied with Raman amplification is considered. If it is assumed that, for transmission conditions in this case, for example, the transmission distance is 250 km, the loss after deterioration over time in the optical cable used for the optical transmission path is 0.190 dB/km, the insertion loss after the breakage of the optical cable is 3.2 dB, and the loss in equipment configuring the system and splice loss is 7 dB, the sum of the losses from the transmitting section to the receiving section is 0.190 dB/km×250 km+3.2 dB+7 dB=57.7 dB. Accordingly, if the power of the supervisory control signal light sent out from the transmitting section after the occurrence of line fault is +7 dBm, the transmission rate is 1.5 Mbps, and the wavelength is 1575±10 nm, the power of the supervisory control signal light that reaches the receiving section in a state where the supply of the pumping light for Raman amplification is stopped, drops to about −51 dBm.
Furthermore, in the case of the unrepeatered transmission system to which optical amplification of remote pumping system is applied, since the optical amplification medium (for example, erbium doped fiber or the like) disposed on the optical transmission path serves as an absorption (loss) medium at the time of non-pumping, the power of the supervisory control signal light that reaches the receiving section drops further. To be specific, if it is assumed that, for the transmission conditions, for example, the transmission distance is 400 km, the loss after deterioration over time in the optical cable (low loss type) used for the optical transmission path is 0.180 dB/km, the insertion loss after the breakage of the optical cable is 3.2 dB, the loss in equipment configuring the system and splice loss is 7 dB, and the absorption by the optical amplification medium is 15 dB, the sum of the losses from the transmitting section to the receiving section is 0.180 dB/km×400 km+3.2 dB+7 dB+15 dB=97.2 dB. Accordingly, when a supervisory control signal light in a condition same as that in the case of the above-described Raman amplification, is sent out from the transmitting section after the occurrence of line fault, the power of the supervisory control signal light that reaches the receiving section, drops to about −90.2 dBm.
In order to receive reliably a supervisory control signal light of weak power as described above by the receiving section, a considerably high-sensitive receiving circuit is required, which is extremely difficult to be realized, and even if it can be realized, the system costs become high.
As one of methods for reducing the average power of a supervisory control signal light sent out from the transmitting section when a line fault occurs, a pulse shaped signal with a narrow width can be considered to be used as a supervisory control signal light. However, such a pulse shaped signal light has the high peak power, so even if it satisfies the safelight level standards, it cannot be recommended from the point of view of protecting human body.